Magnetic resonance (MR) imaging presents a large thermal load. Components that need to be kept cooled include the magnet cryo-compressor (used to maintain liquid helium for cooling the typically superconducting magnet), magnetic field gradient coils, the gradient coil amplifiers, and the radio frequency (RF) amplifier. Other components requiring cooling may also be present.
In a typical medical facility, a commercially available air cooled chiller is used to provide chilled water for cooling the MR imaging device. The chiller may be a dedicated unit, or may be a general-purpose chiller, e.g. a hospital chiller that supplies chilled water for diverse purposes. A liquid cooling cabinet (LCC) provides interfacing between the chilled water and individual cooling systems of the MR imaging device. In a typical arrangement, heat exchangers thermally couple the chilled water to various coolant circuits that cool individual MR components. The coolant circuits are usually isolated from the chilled water, and the various coolant circuits may employ water or other fluids as the coolant. The coolant is maintained at a controlled temperature using a mixing valve to mix “cold” coolant from the heat exchanger and “hot” coolant delivered by a coolant return line from the cooled MR component. The coolant is driven by natural circulation or actively using pumps. In some designs, the cryo-compressor is directly cooled by chilled water from the air-cooled chiller, although provision of an isolating heat exchanger may also be used.
In designing the cooling system for an MR imaging device, the chiller capacity is typically chosen to meet the maximum credible heat load under the least favorable credible operating conditions for the chiller. The maximum credible heat load can be determined as follows. An MR imaging examination of a patient typically includes a sequence of MR imaging scans. The sequence is represented for entry into the MR imaging device controller the form of an examination card (“exam card” is used herein; the representation may be referred to by different nomenclatures depending upon the MR imaging device manufacturer/model and/or the medical facility). The heat load produced by the MR imaging device depends on the scan type of the scan being executed and its parameters. For example, one common metric of heat load is the root-mean-square magnetic field gradient field (Grms), which is affected by various parameters of the MR scan. A higher value for Grms will impose a higher heat load. In general, most of the scans have a low heat load, some a moderate and just a small number of scans have a high heat load. Other illustrative metrics of the heat load produced by an MR scan include the frequency spectrum of the magnetic field gradients (since the magnetic gradient coil impedance is frequency-dependent) and the B1,rms field generated by the RF excitation. The maximum credible heat load can be determined assuming worst-case values (from the heat load viewpoint) for the various scan parameters and metrics, e.g. highest Grms, highest gradient frequency content, highest RF pulse energy, and so forth. The exam card definition and the patient workflow can also affect the heat load, and again worst-case values are assumed, e.g. a contiguously performing a series of consecutive maximum-load scans.
The principal operating condition parameter for an air-cooled chiller is the temperature of the ambient air, i.e. the outdoor air temperature for the usual case in which the chiller is installed outdoors. The cooling capacity of a chiller is a function of the outdoor air temperature and the water temperature: The lower the outdoor air temperature, the higher the cooling capacity and the higher the water temperature, the higher the cooling capacity. The nominal cooling capacity of the chiller is usually defined at a design-basis worst case outdoor air temperature and at a default water temperature, e.g. 12° C. During most of the year the outdoor air temperature is significantly lower than the design-basis worst case temperature at which the chiller nominal cooling capacity is defined. There is a spread in the actual cooling capacity as function of the outdoor temperature.
If the chiller capacity is overloaded, then a chain of events is initiated which eventually leads to an MR imaging scan abort. Initially, as the overloaded chiller is unable to handle the heat load the chilled water will begin to increase in temperature. The cooling capacity of the chiller is higher with a higher water temperature, which initially counters this temperature increase; thus, the chilled water temperature increases until a new equilibrium is reached. There is also some margin in the cooling chain (i.e. the coolant circuits providing cooling to the individual MR imaging device components). The temperature of the chilled water supplied by the chiller can increase somewhat in temperature before coolant circuits cannot hold the temperature of the coolant, and moreover the coolant can increase in temperature by some amount before the cooled MR imaging device component overheats and a thermal interlock is triggered that aborts the MR imaging scan. But eventually, if the chiller overload continues unabated, the coolant temperature for some component (or the temperature of the component itself) exceeds its interlock threshold and triggers a scan abort.
Since scan aborts are undesirable, the design-basis maximum load and maximum outdoor temperature values are usually chosen to be so large as to essentially eliminate the possibility of a scan abort due to thermal runaway. However, this robustness against scan aborts is obtained at the cost of employing a high-capacity chiller, which increases the initial cost of the MR imaging device installation, and increases maintenance and replacement costs down the line. MR imaging facility operational expenses are also impacted as a higher-capacity chiller uses more energy to operate.
The following discloses a new and improved systems and methods that address the above referenced issues, and others.